1. Field of the Invention
This invention relates to vehicular radar systems, and more particularly to a vehicular collision avoidance radar system using digital signal processing techniques.
2. Description of Related Art
There is a continuing need to increase the density of vehicles traveling the world's roadways, and simultaneously to improve the safety of highway vehicle operations by preventing highway vehicles from colliding with stationary and moving objects (such as roadside obstacles and other vehicles). One means for accomplishing these seemingly contradictory goals is to monitor the relative speed, direction of travel, and distance between vehicles sharing the roadway, and to use such information to provide direct indications to the vehicle's operator of potential danger. It is becoming increasingly more common for automotive engineers to consider the use of microwave radar systems as a means to monitor such environmental conditions.
Vehicle borne radar systems which transmit and receive at three different frequencies on a time division basis, with two of the frequencies being used to determine range, and the third being combined with one of the first two to determine closing speed and likelihood of collision, are presently known. One such system is disclosed in U.S. Pat. No. 3,952,303 to Watanabe et al., which teaches an analog radar signal processing front end.
However, analog systems such as the one disclosed in Watanabe are sensitive to temperature changes, and are difficult to calibrate. Furthermore, such systems are dedicated to particular tasks, such as determining the range and relative rate of motion of other objects, and therefore are difficult to upgrade and customize to meet varying requirements. Still further, the transmit and receive frames in such three frequency systems can be wasteful, in that only small portions thereof are needed to determine the range and relative rate of motion of a target, with the remaining portions of the frame being unused.
Another recent example of an automotive radar system that uses analog signal processing techniques to analyze reflected radar signals is described in U.S. patent application, Ser. No. 08/020,680, entitled Multi-Frequency Automotive Radar System, and assigned to the assignee of the present invention. In that system, a transmit signal and the reflected received signal are coupled to an RF mixer. The relevant output from the RF mixer is a signal that has a frequency equal to the difference between the transmit and receive frequencies. The frequency of the reflected received signal may be shifted from the frequency of the transmit signal upon its return due to the "Doppler" effect. Doppler effect occurs whenever a transmitted signal reflects off a target that has a motion relative to a transceiver. The resulting frequency shift is referred to as a "Doppler shift".
The transmit signal changes at regular intervals between three frequencies spaced 250 kHz apart. Two of the frequencies are used to generate range information as described therein, while a third frequency is used to determine Doppler closing rate and target selection. After substantial analog waveform detection, amplification, shaping, and gating, the information regarding range, closing rate, and target selection can be input to a microcontroller for digital processing.
The use of analog processing techniques is fast and allows real time processing. However, the cost of analog circuitry is typically much greater than the cost of digital circuitry. Thus, the sooner the analog signal can be converted to a digital signal and handled by digital circuitry, the lower the cost of the system. Additionally, digit signal processing circuits are much less sensitive to temperature and manufacturing variations and interference from noise than are analog signal processing circuits. Furthermore, the use of analog signal processing techniques limits the number of features that can be added to a system since each new feature typically requires all new processing hardware. In contrast, many additional features can be added to a system in which digital signal processing is used to determine range and relative motion simply by adding software. Still further, in analog systems the level of sophistication that can be achieved is limited by the available hardware and the cost of such hardware.
Because only a small part of the reflected signal is returned to the antenna, target detection runs from very good to non-existent, even when a strongly reflecting target is present. Improving the ability to detect targets requires sophisticated signal processing. Under many circumstances, such sophisticated signal processing is the only means by which meaningful information can be attained. Without sophisticated information processing it may be difficult to identify and interpret the reflected signal. This level of processing essentially mandates digital signal manipulation.
Therefore, there is a need for an automotive radar system that converts signals received into digital form before processing of those signals. Furthermore, there is a need for a simplified system in which only two frequencies are broadcast and in which a larger portion of the transmit signal is useful.